Character viewing and reproducing apparatus for a photocomposing machine

ABSTRACT

Light pulses in the form of selected character images are retained by a luminescent layer which is activated by the illuminated portion of a superimposed photoconductive layer onto which the character images are projected. Due to feedback from the luminescent layer to the photoconductive layer, the character image is retained for viewing as long as a voltage is applied to the superimposed layers. The character is also projected onto a record carrier which receives a light pulse only when a shutter is operated after the correctness of the selected character has been determined by viewing the character image, which is then erased.

United States Patent Inventors Dieter Sobottka Middelstahr; Rolf Gruner,Jever, both of Germany Appl. No. 835,099 Filed June 20, I969 PatentedDec. 14, 1971 Assignee Olympia Werke Aktiengesellschait Wilhelmshaven,Germany Priority June 22, 1968 Germany P 17 61 675.0

CHARACTER VIEWING AND REPRODUCING APPARATUS FOR A PHOTOCOMPOSING MACHINE16 Claims, 7 Drawing Figs.

Int. Cl B41b 15/08, '7 B4lb25/24,H01j 31/50 Field of Search 95/4.5;

[ 56] References Cited UNITED STATES PATENTS 3,059,i 18 10/1962 Koury250/213 3,l63,096 12/1964 Noble 95/45 UX 3,327,122 6/1967 Dueker et al.250/213 3,330,191 7/1967 King et al 95/4.5 3,489,482 i/l970 Brill 350/96B Primary Examiner-Samuel S. Matthews Assistant Examiner-Robert P.Greiner Attorney-Michael S. Striker ABSTRACT: Light pulses in the formof selected character images are retained by a luminescent layer whichis activated by the illuminated portion of a superimposedphotoconductive layer onto which the character images are projected. Dueto feedback from the luminescent layer to the photoconductive layer, thecharacter image is retained for viewing as long as a voltage is appliedto the superimposed layers. The character is also projected onto arecord carrier which receives a light pulse only when a shutter isoperated after the correctness of the selected character has beendetermined by viewing the character image, which is then erased.

Patented Dec. 14, 1971 3 Sheets-Sheet 1 I INVENTOR S .D/ 17 8: J'oh' fiATTORNEY I Patented Dec. 14,1971

3 Sheets-Sheet I BY 0 llkw' ATTORNEY CHARACTER VIEWING AND REPRODUCINGAPPARATUS FOR A PHOTOCOMPOSING MACHINE BACKGROUND OF THE INVENTION Thepresent invention relates to optical typewriters and photocomposingmachines in which selector means, in the form of manually operated keys,efiect the selection of characters which are projected onto a recordcarrier to produce recorded characters on the same.

In photocomposing typewriters it is desired that the recorded text canbe simultaneously viewed so that errors can be detected, and the lengthof the lines determined before the characters are recorded on the recordcarrier.

Photocomposing machines are known which expose an electrostaticallycharged record carrier, for example a zinc oxide paper. Electricalcharges are dissipated from illuminated portions of the record carrierso that only the unexposed portions remain electrically charged, and canbe made legible in a developing and toning device which makes thecharacters appear on the record carrier. In accordance with the priorart, it is not possible to read the electrostatically recordedcharacters before the same are permanently fixed.

SUMMARY OF THE INVENTION It is one object of the invention to overcomethis disadvantage of known optical typewriters and photocomposingmachines, and to provide a photoelectric storage and viewing devicewhich retains character images projected onto the same for viewing bythe operator, until the storage and viewing device is deenergized, and acharacter reproduced.

Another object of the invention is to provide a photoelectric storageand viewing device which consists of a plurality of photoamplifierelements arranged in crossing lines and being substantially smaller thanthe character images projected thereon.

Another object of the invention is to provide a storage and viewingdevice whose height corresponds to at least one character, and whosewidth corresponds to a plurality of characters, for example to thelength of a line of characters.

Another object of the invention is to provide a storage and viewingdevice which retains a stored character image for an indefinite time,until it is erased by the operator.

Another object of the invention is to retain character images byfeedback from luminescent elements to photoconductive elements of thephotoelectric storage and viewing device.

With these objects in view, an apparatus according to the invention,which is particularly suited for photocomposing machines and opticaltypewriters, comprises an alpha-numerical series of selector means, suchas typewriter keys; character reproducing means operated by thecharacter keys to project first light pulses in the form of selectedcharacter images onto a record carries through a shutter; aphotoelectric storage and viewing device illuminated by successivesecond light pulses representing the same character images andretaining, while energized, the character images for viewing aftertermination of the respective light pulses; and control means, includingcircuit means and switches, for connecting terminals, preferably in theform of plates, with a voltage during successive operations of thecharacter keys.

As a result, the retained character images can be viewed for a desiredtime until the shutter has been operated to reproduce the character on arecord carrier, whereupon the voltage is disconnected from theterminals, and the storage and viewing device is deenergized and thestored character images erased.

The height of the viewing area of the storage and viewing devicecorresponds to at least one character, and its width to a plurality ofcharacters, which may correspond to a word of average length, or to anentire line.

In the preferred embodiment of the invention, the storage and viewingdevice includes a plurality of photoamplifier elements arranged incrossing lines, and each photoamplifier element includes aphotoconductive element and a luminescent element. The photoconductiveelements form a first layer and the luminescent elements form a secondlayer superimposed on the first layer. When the character reproducingmeans under the control of the keys project successively differentcharacter images onto a plurality of the photoconductive elements, theelectric resistance of the same is reduced, and consequently therespective luminescent elements are energized and emit light in the formof viewable character images. The light produced by each energizedluminescent element again reduces the resistance of the respectivephotoconductive element after the light pulse representing a selectedcharacter has ended, so that the luminescent elements remain energizedfor an indefinite time period, and until the voltage is disconnectedfrom the terminals of the device by opening a switch.

Preferably, a diaphragm sheet is located between the photoconductivelayer and the luminescent layer which has an aperture for eachphotoamplifier element so that the amount of fed back light can becontrolled. It is also possible to use a diaphragm which is translucentto a desired degree so that the amount of fed back light is alsocontrolled. The novel features which are considered as characteristicfor the invention are set forth in particular in the appended claims.The invention itself, however, both as to its construction and itsmethod of operation, together with additional objects and advantagesthereof, will be best understood from the following description ofspecific embodiments when read in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective viewillustrating an optical typewriter provided with a character viewingapparatus in accordance with the invention;

FIG. 2 is a fragmentary perspective rear view of a photoelectric storageand viewing device in accordance with one embodiment of the invention,and also including a schematic circuit diagram;

FIG. 3 is a fragmentary perspective front view of the embodiment shownin FIG. 2;

FIG. 4 is partly a circuit diagram, and partly a fragmentary perspectiveview of a storage and viewing device in accordance with anotherembodiment of the invention;

FIG. 5 is a fragmentary cross sectional view of a storage and viewingdevice according to the invention shown on a larger scale;

FIG. 5a is a fragmentary cross-sectional view illustrating amodification of the diaphragm shown in FIG. 5 and FIG. 6 is afragmentary exploded perspective view illustrating another embodiment ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, an opticaltypewriter 2 has character selector means in the form of a keyboard 3whose keys 3a are respectively associated with alpha-numericalcharacters. A space bar 3b is also provided.

The typewriter 2 has transverse guide rails 2a on which a carriage 4 ismounted for movement along a record carrier sheet 5 and a platen, ofwhich only the knob 2b is shown. Upon operation of any character key 3a,or of the space bar 3b, an escapement mechanism of conventionalconstruction,

not shown, effects stepwise movement of the carriage 4 across the recordcarrier sheet 5.

Carriage 4 is a closed casing in which character reproducing means asdescribed in US. Pat. No. 3,228,313 are mounted, and such characterreproducing means may include a lamp and templates having the outline ofcharacters so that light pulses in the form of character images areproduced. An optical guide means 12, which is preferably made ofparallel light conductive glass fibers, guides and projects thecharacter images onto the photoelectric storage and viewing device 10,while a corresponding light guiding and projecting means 12a projectscharacter images onto the record carrier sheet 5 which is made sensitiveto light so that the character images can be permanently fixed on therecord carrier sheet 5 in a well known manner which is not an object ofthe inventionA photoelectric storage and viewing device is shown inFIGS. 2, 3 and 5. The device has a transparent terminal plate in therear, and a transparent terminal plate 16 in front which areelectrically connected by a switch 21 to a voltage source 20.

The photoelectric storage and viewing device 10 is composed of a greatnumber of photoamplifier elements 11 arranged in crossing lines, eachphotoamplifier element 11 being substantially smaller than a characterimage 4a projected by light guiding means 12, and than a character image4b projected by light guiding means 12 0 onto the record carrier sheet5. Each photoamplifier element 11 includes a photoconductive elements 13a and a luminescent element 14 a. The photoconductive element 13 a forma photoconductive rear layer 13, and the luminescent elements 14 a forma luminescent front layer 14.*( *described in Joumal of the OpticalSociety of America Volume 44, No. 4, Apr., 1954.) However, thephotoamplifiers 11 are separated from each other in the two layers bycrossing insulating strips 11a and 1 lb.

When the keys 3a are operated, the character reproducing means in casing4 project pulses representing character images through light guidingmeans 12 onto the transparent conductive terminal 15 through the sameonto the photoconductive layer 13, illuminating a limited number ofphotoconductive elements 13a forming the outline of the character 4ashown to be a E. If the intensity of the light is sufficient, theresistance of the photoconductive elements 13a forming the characterimage 40 is reduced so that the voltage is applied through the closedswitch 21 to the terminal plates 15 and 16 and the respective conductiveelements 130 whereby the luminescent elements 14a forming the outline ofthe character image 4a are energized and activated to produce light sothat the character image 4a is visible in front of the storage andviewing device 10. Since terminal 16 is transparent, viewing of theluminescent layer 14 is possible. A voltage of about 400 Hertz having250 volts is suitable.

Only a short light pulse is produced when a key is operated andreleased, but the character image 4a formed on the luminescent layer 14is nevertheless retained since the activated luminescent elements 14ailluminate the respective photoconductive elements 13 a also forming theoutline of the character image 4a, so that the resistance of therespective photoconductive elements remains low, and the voltagecontinues to pass through the respective luminescent elements 140 tomaintain the same in an active condition. Due to this feedback, which isindicated by arrows in FIG. 5, the character image 4a is retained forviewing after termination of the light pulse produced by operation of akey 30 and of the character reproducing means in casing 4.

The size of the photoamplifier elements 11a, and of the respectivephotoconductive elements 13a and luminescent elements 140, is selectedso that each photoconductive element 13a is associated with a lightguiding filament of the light guiding means 12. Due to the small size ofthe photoamplifier elements 11, a good resolution of the characterimages 4a is obtained so that the same are easily legible.

Between the photoconductive layer 13 and the luminescent layer 14, adiaphragm plate 17 is disposed by which the amount of light fed backfrom luminescent elements 140 to photoconductive elements 13a iscontrolled. In one embodiment of the diaphragm, the same is translucent,and is constructed to be just thick enough to permit a suitable amountof light to pass therethrough from a luminescent element to aphotoconductive element, as indicated by arrows in FIG. 5. It is alsopossible to use a colored translucent diaphragm. In another embodiment,a diaphragm 17 has an aperture between each pair of photoconductive andluminescent elements 13a, 14a forming a photoamplifier element 11, andthe size of the aperture 17a is selected to obtain the desired amount offeedback from the luminescent element 14a.

When the keys 3a are successively actuated for short time periods andthen released, the escapement mechanism, not

shown, causes stepwise movement of carriage 4 with the characterreproducing means, not shown, along the storage and viewing device 10 sothat character images are projected onto the same in adjacent positionsto first form words, and then an entire line. Due to the above explainedfeedback, the character images 4a are retained after termination of thelight pulse produced under the control of a key 3a, and all charactersof a word or of a line appear visible on the storage and viewing device10. In the embodiment of FIG. 1, the size of the character images 4a isselected so that only a limited number corresponding to one or two wordsappears on the viewing device, but evidently it is possible to usesmaller character images and to view an entire line of character images.When the operator is satisfied that the characters appearing on thestorage and viewing device 10 contain no error, the switch 21 isoperated, and the storage and viewing device is deenergized so that theluminescent elements 14a no longer produce light, and the characterimages 4a are erased.

As shown in FIG. 4, a character storage and viewing device 10' mayconsist of a plurality of storing and viewing means to 10f, for example,each of which comprises a plurality of photoamplifiers 11, not shown inFIG. 4, but constructed and arranged as described with reference toFIGS. 2, 3 and 5. Each storage and viewing means 10a to 10f has asufficient size for the viewing of several character images. Theadvantage of the embodiment of FIG. 4 is that small groups of characterimages can be erased by operation of switches 21a to 21f withoutrequiring erasing of the entire line, or of the entire text. Aftererasing of the character image in one of the storage and viewing means10a to 10f, the respective portion of the text is again typed in theproper position by operation of keys 3a so that the character imagesappear on the respective storage and viewing means 10a to 10f.

FIG. 6 schematically illustrates another embodiment of the inventionused with a typewriter as described with reference to FIG. 1. The lightguiding means 12 consists of parallel light guiding glass filaments sothat the selected character image is projected onto the storage andviewing device 10 and retained by the same for an indefinite perioduntil a switch 21 is opened. FIG. 6 also shows other positions 12' ofthe stepwise moving light guiding means 12.

The character images appearing on the luminescent layer 14 and visiblethrough the transparent terminal plate 16 are projected onto asemitransparent mirror 23 which directs the light partly to a reflectingsurface 24 by which the character images are projected onto a frostedglass plate 25 for viewing. After the operator has checked thecorrectness of the character images appearing on the frosted glassplates 25, a shutter means 27 is opened to permit passage of the lightpassing through the semitransparent mirror 23 to the record carriersheet 5 where the light produces character images. A shutter 27 may alsobe provided in the embodiment of FIG. 1 between the light guiding means12a and the record carrier sheet 5 so that permanent printing takesplace only after the correctness of the text appearing on the storageand viewing device 10 has been determined by the operator.

The control of the recorded character images may also be effected byusing a fully reflecting mirror substantially in the position of mirror23 in FIG. 6. The reflecting mirror has a position in which thecharacter images are projected onto the frosted glass plate 25 forviewing, as described with reference to FIG. 6. When the viewed text hasbeen approved, the reflecting mirror is removed so that the characterimages on the storage and viewing device 10 directly act at the openshutter 27 to expose a corresponding portion of record carrier sheet 5.

A storage and viewing device 10 is preferably manufactured bysuperimposing on the transparent electrically conducting plate 15 thephotoconductive layer 13, the transparent and conductive diaphragm 17,the luminescent layer 14, and finally the transparent and electricallyconducting plate 16.

During the application of the photoconductive layer and of theluminescent layer, grid-shaped screens are used for obtaining thedivision of the layers into photoconductive elements and luminescentelements.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofcharacter viewing apparatus for photocomposing machines differing fromthe types described above.

While the invention has been illustrated and described as embodied in aphotoelectric storage and viewing device having a luminescent layer andphotoconductive layer composed of photoamplifying elements, it is notintended to be limited to the details shown, since various modificationsand structural changes may be made without departing in any way from thespirit of the present invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

1. Character viewing and reproducing apparatus comprising a series ofselector means representing a series of alpha-numerical characters;character reproducing means operated by said selector means to transmitlight pulses in the shape of selected character images of said seriesalong first and second paths; supporting means for supporting aphotosensitive record carrier sheet in said first path between saidsupporting means with said record carrier sheet and said characterreproducing means; a photoelectric storage and viewing device havingterminals for being energized, and located in said second path so as tobe illuminated by light pulses representing selected character images,said storage and viewing device retaining, while energized, saidcharacter images for viewing after termination of the respective lightpulses; and control means for connecting said terminals with a voltageduring successive operations of said selector means so that thecharacter images can be viewed for a desired time whereby after saidviewed character images have been found correct, said shutter can beoperated to effecton said record carrier sheet, photographicreproduction of selected character images projected along said firstpath while said control means connects said terminals with said voltage;said control means being operable to disconnect said voltage from saidterminals so that said storage and viewing device is deenergized andsaid retained characters are erased 'after reproduction on said recordcarrier sheet.

2. An apparatus as claimed in claim 1 wherein said control means includemanually operated switch means.

3. An apparatus as claimed in claim 1 wherein said character reproducingmeans is located between said supporting means and said storage andviewing device, and projects said light pulses in opposite directionstoward a record carrier sheet on said supporting means and toward saidstorage and viewing device.

4. An apparatus as claimed in claim 1 wherein said first and secondpaths are aligned.

5. An apparatus as claimed in claim 1 wherein said selector meansinclude a keyboard with keys associated with the characters of saidseries; and wherein said storage and viewing device is located rearwardof said keyboard.

6. An apparatus as claimed in claim 1 wherein said control means includemanually operated switch means; wherein said character reproducing meansis located between said supporting means and said storage and viewingdevice, and projects said light pulses in opposite directions toward arecord carrier sheet on said supporting means and toward said storageand viewing device; wherein said first and second paths are aligned;wherein said selector means include a keyboard with keys associated withthe characters of said series; and wherein said storage and viewingdevice is located rearward of said keyboard and forward of saidcharacter reproducing means.

7. Apparatus as claimed in claim 1 wherein said storage, and

character image, and a width at least equal to the width of a pluralityof said character images 8. Apparatus as claimed in claim 1 comprising asemitransparent mirror located between said storage and viewing deviceand said shutter; reflecting means; and a frosted glass plate disposedso that light reflected by said semitransparent mirror is deflected bysaid reflector means onto said frosted glass plate for observation ofthe character images retained by said photoelectric storage and viewingdevice.

9. Apparatus as claimed in claim 1 wherein said storage and viewingdevice includes a plurality of storageand viewing means arranged in acommon viewing plane; wherein each storage and viewing means has a pairof terminals connected with said control means; and wherein said controlmeans includes switches operable for selectively energizing anddeenergizing each of said storage and viewing devices.

10. Apparatus as claimed in claim 1 wherein said character reproducingmeans include light guiding means consisting of parallel glass fibers;and wherein the resolution of said glass fibers correspondssubstantially to the resolution of said storage and viewing device, eachcharacter image being composed of light points respectively associatedwith said glass fibers.

11. Apparatus as claimed in claim 1 wherein said storage and viewingdevice includes a photoconductive layer, a luminescent layer, andterminals in the form of light permeable terminal plates located onopposite sides of the superimposed photoconductive and luminescentlayers.

12. Apparatus as claimed in claim 1 wherein said storage and viewingdevice includes a plurality of photoamplifier elements arranged incrossing lines, each photoamplifier element including a photoconductiveelement and a luminescent element, said photoconductive elements forminga first layer and said luminescent elements forming a second layersuperimposed on said first layer; wherein said character reproducingmeans projects each character image onto a plurality of saidphotoconductive elements whereby the electric resistance of the same isreduced; and wherein said terminals are transparent terminal platesbetween which said first and second layers are located so thatilluminated photoconductive elements cause energization of therespective luminescent elements which feed back light to the respectivephotoconductive elements after termination of said light pulses wherebyviewable character images are retained by energized luminescent elementsuntil said voltage is disconnected from said transparent terminal platesby said control means.

13. Apparatus as claimed in claim 12 wherein said character reproducingmeans include light guiding means having a plurality of parallel glassfibers disposed in crossing lines and having a size and positioncorresponding to the size and position of said photoconductive elementsso that points of each character image are transmitted from said glassfibers to corresponding photoamplifier elements of said storage andviewing device.

14. Apparatus as claimed in claim 12 wherein said storage and viewingdevice includes a diaphragm sheet located between said first and secondlayers for limiting the amount of light fed back from said luminescentelements to said photoconductive elements.

15. Apparatus as claimed in claim 14 wherein said diaphragm sheet istranslucent.

16. Apparatus as claimed in claim 14 wherein said diaphragm sheet has anaperture located between each pair of luminescent and photoconductiveelements forming a photoamplifier element, the size of said aperturebeing selected to determine the amount of light fed back from saidluminescent elements to said photoconductive elements.

1. Character viewing and reproducing apparatus comprising a series ofselector means representing a series of alpha-numerical characters;character reproducing means operated by said selector means to transmitlight pulses in the shape of selected character images of said seriesalong first and second paths; supporting means for supporting aphotosensitive record carrier sheet in said first path between saidsupporting means with said record carrier sheet and said characterreproducing means; a photoelectric storage and viewing device havingterminals for being energized, and located in said second path so as tobe illuminated by light pulses representing selected character images,said storage and viewing device retaining, while energized, saidcharacter images for viewing after termination of the respective lightpulses; and control means for connecting said terminals with a voltageduring successive operatioNs of said selector means so that thecharacter images can be viewed for a desired time whereby after saidviewed character images have been found correct, said shutter can beoperated to effect on said record carrier sheet, photographicreproduction of selected character images projected along said firstpath while said control means connects said terminals with said voltage;said control means being operable to disconnect said voltage from saidterminals so that said storage and viewing device is deenergized andsaid retained characters are erased after reproduction on said recordcarrier sheet.
 2. An apparatus as claimed in claim 1 wherein saidcontrol means include manually operated switch means.
 3. An apparatus asclaimed in claim 1 wherein said character reproducing means is locatedbetween said supporting means and said storage and viewing device, andprojects said light pulses in opposite directions toward a recordcarrier sheet on said supporting means and toward said storage andviewing device.
 4. An apparatus as claimed in claim 1 wherein said firstand second paths are aligned.
 5. An apparatus as claimed in claim 1wherein said selector means include a keyboard with keys associated withthe characters of said series; and wherein said storage and viewingdevice is located rearward of said keyboard.
 6. An apparatus as claimedin claim 1 wherein said control means include manually operated switchmeans; wherein said character reproducing means is located between saidsupporting means and said storage and viewing device, and projects saidlight pulses in opposite directions toward a record carrier sheet onsaid supporting means and toward said storage and viewing device;wherein said first and second paths are aligned; wherein said selectormeans include a keyboard with keys associated with the characters ofsaid series; and wherein said storage and viewing device is locatedrearward of said keyboard and forward of said character reproducingmeans.
 7. Apparatus as claimed in claim 1 wherein said storage andviewing device has a height at least equal to the height of onecharacter image, and a width at least equal to the width of a pluralityof said character images.
 8. Apparatus as claimed in claim 1 comprisinga semitransparent mirror located between said storage and viewing deviceand said shutter; reflecting means; and a frosted glass plate disposedso that light reflected by said semitransparent mirror is deflected bysaid reflector means onto said frosted glass plate for observation ofthe character images retained by said photoelectric storage and viewingdevice.
 9. Apparatus as claimed in claim 1 wherein said storage andviewing device includes a plurality of storage and viewing meansarranged in a common viewing plane; wherein each storage and viewingmeans has a pair of terminals connected with said control means; andwherein said control means includes switches operable for selectivelyenergizing and deenergizing each of said storage and viewing devices.10. Apparatus as claimed in claim 1 wherein said character reproducingmeans include light guiding means consisting of parallel glass fibers;and wherein the resolution of said glass fibers correspondssubstantially to the resolution of said storage and viewing device, eachcharacter image being composed of light points respectively associatedwith said glass fibers.
 11. Apparatus as claimed in claim 1 wherein saidstorage and viewing device includes a photoconductive layer, aluminescent layer, and terminals in the form of light permeable terminalplates located on opposite sides of the superimposed photoconductive andluminescent layers.
 12. Apparatus as claimed in claim 1 wherein saidstorage and viewing device includes a plurality of photoamplifierelements arranged in crossing lines, each photoamplifier elementincluding a photoconductive element and a luminescent element, saidphotoconductive elements forming a first layer and said luminesceNtelements forming a second layer superimposed on said first layer;wherein said character reproducing means projects each character imageonto a plurality of said photoconductive elements whereby the electricresistance of the same is reduced; and wherein said terminals aretransparent terminal plates between which said first and second layersare located so that illuminated photoconductive elements causeenergization of the respective luminescent elements which feed backlight to the respective photoconductive elements after termination ofsaid light pulses whereby viewable character images are retained byenergized luminescent elements until said voltage is disconnected fromsaid transparent terminal plates by said control means.
 13. Apparatus asclaimed in claim 12 wherein said character reproducing means includelight guiding means having a plurality of parallel glass fibers disposedin crossing lines and having a size and position corresponding to thesize and position of said photoconductive elements so that points ofeach character image are transmitted from said glass fibers tocorresponding photoamplifier elements of said storage and viewingdevice.
 14. Apparatus as claimed in claim 12 wherein said storage andviewing device includes a diaphragm sheet located between said first andsecond layers for limiting the amount of light fed back from saidluminescent elements to said photoconductive elements.
 15. Apparatus asclaimed in claim 14 wherein said diaphragm sheet is translucent. 16.Apparatus as claimed in claim 14 wherein said diaphragm sheet has anaperture located between each pair of luminescent and photoconductiveelements forming a photoamplifier element, the size of said aperturebeing selected to determine the amount of light fed back from saidluminescent elements to said photoconductive elements.